Fix Router::merge for overlapping routes same different methods (#543)

Discovered while working on something else that we didn't properly
handle this:

```rust
let one = Router::new().route("/", get(|| async {}));
let two = Router::new().route("/", post(|| async {}));

let app = one.merge(two);
```

axum/src/routing/mod.rs

@@ -244,12 +244,15 @@ where
             nested_at_root,
         } = other;
 
-        if let Err(err) = self.node.merge(node) {
-            self.panic_on_matchit_error(err);
-        }
-
         for (id, route) in routes {
-            assert!(self.routes.insert(id, route).is_none());
+            let path = node
+                .route_id_to_path
+                .get(&id)
+                .expect("no path for route id. This is a bug in axum. Please file an issue");
+            self = match route {
+                Endpoint::MethodRouter(route) => self.route(path, route),
+                Endpoint::Route(route) => self.route(path, route),
+            };
         }
 
         self.fallback = match (self.fallback, fallback) {
@@ -603,13 +606,6 @@ impl Node {
         Ok(())
     }
 
-    fn merge(&mut self, other: Node) -> Result<(), matchit::InsertError> {
-        for (id, path) in other.route_id_to_path {
-            self.insert(&*path, id)?;
-        }
-        Ok(())
-    }
-
     fn at<'n, 'p>(
         &'n self,
         path: &'p str,

axum/src/routing/tests/mod.rs

@@ -584,3 +584,19 @@ async fn nesting_router_with_fallbacks_panics() {
     let app = Router::new().nest("/", one);
     TestClient::new(app);
 }
+
+#[tokio::test]
+async fn merging_routers_with_same_paths_but_different_methods() {
+    let one = Router::new().route("/", get(|| async { "GET" }));
+    let two = Router::new().route("/", post(|| async { "POST" }));
+
+    let client = TestClient::new(one.merge(two));
+
+    let res = client.get("/").send().await;
+    let body = res.text().await;
+    assert_eq!(body, "GET");
+
+    let res = client.post("/").send().await;
+    let body = res.text().await;
+    assert_eq!(body, "POST");
+}